A blur on a rear-view while parking has always been a problem for drivers, especially under conditions of bad weather and narrow space for parking, and it is thus easy for a collision to occur. One kind of existing parking device uses detecting principles of ultrasonic transmitting and receiving. While reversing a car, a buzzer sounds and a nixie tube indicates the distance of an obstacle without specific conditions of the rear portion of the car and the orientation, size and shape of the obstacle behind the car. For another kind of existing parking device with a camera, drivers are able to know the condition of the rear portion of the car, but not the exact distance of the obstacle, and at the same time the camera has a dead angle. Both of the above parking devices only unilaterally help drivers reverse the car. While reversing the car, the drivers have to judge the distance between the rear portion of the car and the obstacle in a moment, and simultaneously they have to judge the orientation and shape of the obstacle, which increases the difficulty in driving. During installation, a signal emitting device at the rear of the car and a driver monitoring device at the front of the car need to be connected with wires from the back to the front of the car, which increases the difficulty of the installation.